Electrostatic coating processes rely on a charge differential between an article to be coated and what is used to coat that article. In such processes, the article is typically grounded whereas the coating to be applied is endowed with a charge. When the article and coating are then brought into contact with one another, the result is that the coating adheres to the article. It is estimated that more than 10,000 facilities for accomplishing this exist in the US alone.
Most such coating procedures and facilities employ a variety of steps, i.e., a cleaning step, a drying step, a coating step, and a heating step wherein the adhered coating is cured to afford a more desirable and permanent coat. These steps usually take place sequentially using batch operations commonly employed in the art, or else in specialized stations connected by a continuous conveyor line.
Conveyor lines can be of varying length depending on the facility. Articles to be coated are hung from these lines via spaced electroconductive racks or hangers that serve to ground articles attached thereto. Racks and hangers are popular that have the capacity to hang multiple articles. This is accomplished by multiple hooks, usually spot welded at set distances from one another on the same rack. Such rack and hook configurations vary widely in shape, size, and configuration to support different types and sizes of articles.
Once attached, the hangers or racks bearing grounded articles are conveyed through a coating station followed by a curing station. Once coating and curing are finished, the coated objects are removed and the process begins anew.
The hangers and racks of such systems, being expensive, are typically re-used. After passing through the painting station a number of times, that portion or portions of the hanger which contact the article gradually becomes fouled by coating. The net effect is interference with grounding capacity, with consequent poor transfer efficiency and an eventual possibility for spark or fire. This necessitates periodic replacing or cleaning, which is both time-consuming and expensive.
In the case of recycling, conventional cleaning methods include chemical stripping, molten bath stripping, burning, and mechanical stripping, i.e., sandblasting, hammering, and filing. These processes reduce the useful life and capacity of racks by compromising their structural integrity over time. For example, it is the Applicants""experience that hooks break off fairly regularly, thereby lessening the capacity and desirability of continuing with that rack.
The art has thus far failed to provide a cost-effective alternative.
The invention provides a surprisingly efficient solution to the long-felt need described above.
It is an object of the invention to provide an electrically conductive intermediate at an interface or contact point between the hanger and article to be coated. This intermediate may be conveniently replaced or recycled at a comparatively small cost relative to existing procedures and implements.
In a first aspect, the invention features a system for extending the operating life of hangers or racks associated with electrostatic coating. This is accomplished by use of a relatively cheap, electrically conductive, and preferably pliable, intermediate that is suitable for grounding an article to be coated. The intermediate is interposed at a contact junction of the article and electroconductive hanger.
In exemplary embodiments, the intermediate slideably engages, wraps, or clamps to the hanger and may even adapt in shape or be engineered to accommodate the particular shape of a hook. In most preferred embodiments the article, via an orifice or recess, envelops at least a portion of the hook and intermediate attached thereto.
Various embodiments contemplate different conductive materials and configurations, including shape, of the intermediate. By way of materials, rubber, plastic, tape, and metallic foils all exist that are conductive and suitable, depending on the precise application. The intermediate may be a silicone sleeve having a hollow interior for receiving a hook portion of a hanger. The sleeve may be open at both ends or may be a cap having one closed end. The article to be coated then fits over or engages the portion of the hook enveloped by the intermediate, usually via an orifice of sufficient dimension.
Concentric xe2x80x9clayersxe2x80x9d of pliable sleeves are also envisioned for some coating applications wherein one sleeve is positioned over another for rapid exposure of fresh contact surfaces as appropriate. A spent layer is simply peeled away or cut off thereby exposing a fresh one. One such embodiment contemplates a tape. Other embodiments contemplate a plurality of hollow tubes, one over the top of the next. These may be slit lengthwise and deposited one over the top of the next, or else constructed in multiplied layers which are then curled and fixed in form to wrap or clamp to a hanger of interest. Of course, the diameter differential associated with this technique must accordingly be accommodated by the article.
In other embodiments, at least a portion of the hanger itself comprises a nonmetallic material such as a conductive silicone rubber or plastic. This new material can be conductively and integrally fixed during manufacture, e.g., by injection molding. Preferably, the material is pliable or bendable with the hands or other gentle means to quickly release or free unwanted deposits of coating that hinder contact and hence grounding ability. In such embodiments, the sleeve or intermediate is recyclable.
In still other embodiments, the sleeve intermediate is disposable. Of course, everything including hangers are disposable at a cost, but what distinguishes the present invention is the relatively low cost of the intermediate relative to the cost of replacing or recycling a hanger or rack. In embodiments where the intermediate is integrally a part of the hanger, the novelty resides in the hanger being easily cleaned relative to conventional hangers, e.g., metal ones, and more durable or receptive to cleanings.
In exemplary embodiments, the intermediate bridges a hanger and an article to be coated. This bridge may occur in a variety of configurations as one of skill will appreciate. It may occur as described above, or else it may occur by a more comprehensive envelopment, not only of the hanger but also of the entire juncture, including a portion of the article itself. U.S. Pat. No. 5,897,709 issued to Torefors describes one such example. However, instead of a conductive bridge, Torefors specifies a non-conductive (xe2x80x9cdielectricxe2x80x9d) cover. The present invention, by contrast, serves a dual function in further providing a conductive bridge to facilitate grounding and suitable coating, while simultaneously preserving the operative part of the hanger or hook for future use.
In another exemplary embodiment of the invention, an intermediate member is designed for fitting over a horizontal cross-bar type of workpiece hanger which suspends large size panels or the like for electrostatic coating, and comprises a longitudinal, hollow sleeve of pliable, electrically conductive material having a longitudinal slit extending along its length so that the sleeve can be engaged transversely over a cross bar extending between two vertical hangers via the slit. An article to be coated, such as a large flat panel, can then be suspended from the cross bar via conductive hooks which engage over the sleeve.
The elongate sleeve may be of any suitable cross-sectional shape, such as circular, square, rectangular, or octagonal. The slit may form a longitudinal gap or slot in the sleeve, or may be a simple linear cut along the length of the sleeve. Alternatively, the sleeve may have opposite longitudinal edges which are overlapped along the length of the sleeve, so that there is no opening in the sleeve after it has been engaged over the cross bar. In another alternative, the sleeve may have no slit, for engagement over hook like hanger.
In an alternative embodiment, the intermediate may be a sheet or strip of pliable, electrically conductive material which is secured on top of a hanger by an electrically conductive adhesive, such that an article to be coated engages the strip or layer. The pliable strip may have any suitable cross-sectional and peripheral shape, such as square, rectangular, circular, triangular, and the like, and may be solid or may have a through bore. The adhesive may cover all or only part of an inner face of the strip.
The intermediate may suitably be made of a conductive material, preferably rubber, plastic, tape, foil, or grease that can be conveniently removed, disposed of, replaced, or recycled. The intermediate may have resistance of less than 6 megaohms, or one or less megaohms, or 0.5 megaohms, and in one example has a resistance of about 0.1 megaohms or less.
In exemplary embodiments, such intermediates are also heat resistant to temperatures up to 600xc2x0 F., and may be heat resistant in ranges of between about 250xc2x0 F. and 450xc2x0 F.
At present, the favorite known material for the intermediate is conductive silicone, which may be fashioned by mixing different conductive and nonconductive commercially available grades in certain proportions testable by one of skill in the art, using routine experimentation to arrive at a final suitable product. Alternatively, fully conductive commercially available conductive silicone alone can be used that, while more expensive, still represents an improvement in the art.
The material used, e.g., silicone, may be molded to fit the myriad different sizes and shapes of hooks available, or else a universal piece may be used that fits a variety of hook shapes and sizes by conforming pliably in shape. Preferably, these sleeves or caps pull on and off conveniently with minor effort, but are not too loose as to permit undue amounts of coating to seep inside. Looseness is not known to otherwise disadvantage the system, provided there is some contact through which a ground may be established.
A second aspect of the invention features methods for electrostatic coating that make use of the above embodiments, either singularly or, where appropriate, combined. One method of providing an electrostatic pliable coating layer on one or more hanger members comprises dipping at least part of at least one hanger member in a bath of liquid electroconductive material, such as conductive silicone, so that the dipped surface is coated with a layer of electroconductive material, and then lifting the hanger member out of the bath and allowing the coating layer to cure in order to form a pliable, electroconductive coating layer. Some or all of the hanger member may be dipped, and entire hanger racks for use in electrostatically coating many parts at once may be dipped and coated with the pliable electroconductive intermediate.